As a pressure sensor, a sensor is proposed which includes: a metal shell mounted on an engine head; a pressure receiving member including a diaphragm and a pressure receiving rod; a cap screw screwed into the pressure receiving rod; and a piezoelectric sensor interposed between the head of the cap screw and the metal shell. When the diaphragm receives a combustion pressure, the diaphragm is pressed backward, whereby a load is transmitted via the pressure receiving rod to the piezoelectric sensor. The piezoelectric sensor converts a change in the load to a change in electrical output. Here, a heat shielding plate is disposed at the front surface of the diaphragm in order to reduce the amount of thermal deformation of the diaphragm caused by high-temperature combustion gas.